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|Title:||STUDY OF, SAND-SHALE MODELS OF GROUND WATER SYSTEM BY RESISTIVITY INTERPRETATION -A STOCHASTIC APPROACH|
GROUND WATER SYSTEM
EARTH SCIENCE ENGINEERING
|Abstract:||Water resources are divisible into two distinct categories, the surface water resources and the ground water resources. Each of these is part of earth's water circulatory system, which is known as the hydrologic cycle and is derived from precipitation in the form of rain-fall or snow. These are interdependent and very often loss of one is gain of the other. While one can make an assessment to surface water flow which is a visible phenomenon, assessment of ground - water resources is a complex exercise because this is not visible to the naked eye and one has to use various scientific methods and parameters to make an assessment of ground - water resources. There are vast areas which do not have surface water streams to cater the needs of the people and they have to depend mainly on ground - water availability in the region. The need for the developing methodology for assessment of ground - water resources is universal and earth scientists are continuously engaged in developing more and more dependable techniques for assessing these hidden resources. Knowledge of the geology of an area is essential to the understanding of the occurrence of ground - water because -2- the geological structure and stratigraphy provide the frame - work in which ground - water recharge, storage and discharge take place. ilmong~t the sedimentary section of clastic rocks the sand - shale sequences are quite frequently observed in nature. Such sequences have been found to beat times, under suitable conditions of great economic importance particularly for ground - water. The sand - stone in such section provide storage and passage ways for these reservoirs. In such a sequences the higher the percentage of sand the higher amount of water can be expected. Such sequences are generally met in the subsurface with little or no exposure in and around the area of study. Therefore such sub-surface sequences can only be studied by indirect methods. Geophysical methods provide an alternative to the direct methods in such circumstances. The methods of geo-physical exploration can be divided in two major categories. The first includes those techniques which rely on the field, or field of force inherent in the earth itself, natural to the body of the earth or residing in the crust of the planet, the second embraces those methods which depend upon impressing on the earth a foreign field of force, generated and applied by instruments manipulated by the geophysicist. Both categories are employed in ground -. water hydrology, but the second group of artificially applied fields, has the wider and more direct -3- applications. The methods utilizing artifically impressed forces are the electrical and seismic techniques. The elec-trical methods may be subdivided into the resistivity technique and the inductive, or electromagnetic method. The latter has not yet been of importance in hydrological work, although it is highly effective in prospecting for metaOLic minerals. The seismic methods are much costlier as compared to the electrical methods and are used mainly for petroleum exploration. The resistivity methodswhich are much cheaper, are commonly used for ground - water investigation. The field data obtained in the form of an apparent resistivity curve are interpreted and the results are then correlated with available bore-hole infor-mation to arrive at a realistic picture of the subsurface geo-logy vis-a-vis ground - water potentialities of aquifers. The first step is thus termed as physical interpretation and the subsequent procedure as geological interpretation. Procedures in the interpretation are termed as direct or indirect depending upon the manner, the information about the subsurface is derived from the field data. In indirect methods the field curve is compared with a set of precalculated master-curves for known geological conditions of the earth. i. match of the curves is interpreted as a match of the parameters. This method is simple and fast in application but invariably the method fails because an exact fit can not be obtained with 0 the collection of data at hand. Direct method on the other hand depends on the determination of the Kernel function (Slichter, 1933) as an intermediatory step in the process of deriving the layer parameters from field measurements. Koefoed (1970), introduced a new function, called the resistivity transform (related to the Kernel function), and used it as intermediatory function in deriving layer parameters. Ghosh (1971), using the knowledge of sampling and linear filter theory, outlined a quick and efficient method for extracting the resistivity transform from the apparent resistivity data. In the present study an attempt has been made to find a method, to allow the geophysicist to estimate sand - shale ratios of ground - water bearing sequences suited to any given special geohydrological section, with ordinary field facilities and without too great an expenditure of time. This method uses resistivity - transform function for sedimentary formations obtained from the Monte - carlo simulation. 1.2 Review of Previous Work: number of new approaches for mapping stratigraphy using seismic data have been proposed recently. One of the ,most significant parameters that has comes into use for identi-fying lithologies is the seismic interval velocity. Smith (1969), Taner and Koehler (1969), Cook and Taner (1969) , Gardner et al.,(197'+). These interval velocities and their changes have -5- been related to li'fhology. This approach is being used as a regular exploration tool for determining the sand - shale ratio. Savit and Matekar (1971) have proposed the use of seismic energy attenuation as a guide to subsurface lithology. However, this approach is likely to be successful for estimat- ing the gross nature of rather thick sedimentary sections. The statistical analysis of the structure of the seismic reflection to predict lithological changes has been attempted by Mathieu and Rice (1969) and t~vasthi and Verma (1973). Mathieu and Rice (1969) have attempted a discrimina- tory analysis of a linear combination of more than one observed parameter of the seismic trace for the determination of varia-- tion in stratigraphic condition. iivasthi and Verma (1973) have also proposed a multiple parameter approach to the problem of•in±eDring stratigraphy from seismic data, and represented the received signal in the seismogram as a vector in a multiparameter hyper -space. Khattri and Gir (1975, 1976) have used a deterministic approach and have built several models depicting a sand - shale sequence, a coal - shale sequence and basal - sand model, and linked them to transgression-and regression phenomena at the coasts. The approach is essentially concerned with the building - 6 - of mathematical models of the layered earth in order to achieve a better understanding of the seismic responses. The models involved no random elements and were completely deterministic. Variations in sand - shale ratio was achieved by increasing or decreasing the number of sand layers in the model. In prin-ciple this method suggests the possibility of extracting the send - shale ratio from information contained within the seis-mo gram.....|
|Appears in Collections:||MASTERS' DISSERTATIONS (Earth Sci.)|
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